Firefighting foam concentrates contain mixtures of surfactants that act as foaming agents, together with solvents and other additives that provide the desired mechanical and chemical properties for the foam. The concentrates are mixed with water in situ and foamed by mechanical means, and the resulting foam is projected onto the fire, typically onto the surface of a burning liquid. The concentrates are typically used at a concentration of about 1-6%.
Aqueous film-forming foam (AFFF) concentrates are designed to spontaneously spread an aqueous film on the surface of hydrocarbon liquids, which increases the rate at which the fire can be extinguished. This spreading property is made possible by the use of perfluoroalkyl surfactants in AFFF, which produce very low surface tension values in solution (15-20 dynes cm−1), thereby permitting the aqueous solution to spread on the surface of the hydrocarbon liquids.
However, typical AFFF foams are not effective on fires caused by water-miscible fuels, such as low molecular weight alcohols, ketones, and esters and the like, because the miscibility of the fuel and water in the foam leads to dissolution and destruction of the foam by the fuel. To address this issue, alcohol resistant AFFF (AR-AFFF) concentrates are used, which contain a high molecular weight water-soluble polymer that precipitates on contact with a water-miscible fuel, creating a protective layer between the fuel and the foam.
Typical high molecular weight water-soluble polymers used in AR-AFFF are polysaccharides, such as xanthan gums. AR-AFFF foams are effective on both hydrocarbon and water-soluble fuels. However, the high concentrations of polysaccharide gums necessary to make an effective AR-AFFF concentrate can cause the concentrate to be so thick that the concentrate is difficult to pump efficiently, and therefore can cause proportioning problems during foam generation and application. The use of certain fluoropolymer surfactants has the same polar fuel performance as xanthan gums, but with much lower viscosity increase. See, for example, U.S. Pat. No. 6,156,222. Therefore, a significant portion of the gum can be replaced by fluoropolymer surfactants to give better AR-AFFF performance.
Until recently, aqueous film forming foams that were used for fire fighting invariably contained components, including low molecular weight fluorosurfactants and fluoropolymer surfactants having perfluoroalkyl chains where the perfluoroalkyl group was at least a perfluorooctyl group. For example, it was believed that a surfactant required at least a perfluorooctyl moiety to provide the necessary physicochemical attributes for efficient and persistent foam formation for fire fighting applications. See WO03/049813. However, perfluorooctyl moieties have been shown to be environmentally persistent and to accumulate in the livers of test animals, leading to calls for the phase-out of materials, including foam components, containing a perfluorooctyl group. Recent regulatory efforts such as the United States EPA's PFOA Stewardship Program and EC directives pertaining to telomer-based higher homologue perfluorinated surfactants have sought to discourage use of perfluorooctyl-containing components.
Although C6 fluorosurfactants have been reported to be satisfactory for less demanding applications, such as cleaning solutions, the reduction in length of the perfluoroalkyl chain unfortunately leads to a decrease in the ability to form long lasting persistent foams with the properties necessary for effective fire fighting. Thus, AFFF and AR-AFFF concentrates where the perfluorooctyl surfactant is replaced by an equivalent C6 compound typically are unable to meet the requirements of the US and international standards for fire fighting applications.